Flying spot scanning

ABSTRACT

A flying spot scanner system is disclosed which obviates the prior art use of a large expensive lens by reflecting focused light off a multifaceted mirror to the element being scanned. The spot is made invariant and as small in size as possible by defining the dimensions of the focused beam so that only part, preferably half, of a mirror facet is illuminated during scanning; and by locating the mirror as close to the element being scanned as will allow the spot to be formed of all of the light beamed at the mirror. The source of illumination is a laser; and uniform spot intensity during scanning is assured by a suitably dimensioned stop.

United States Patent [15] 3,675,016

Blaisdell et al. July 4, 1972 54] FLYING SPOT SCANNING 3,515,488 6/1970Houser .250/219 DF [72] Inventors: William H. Blaisdell, Rochester;Edward 3469030 9,1969 Pile-hem C Cornelius Fai on both of N Y 2,254,6249/1941 Rrma ..178/7.6 rp 3,517,202 6/1970 Kennedy ..250/2l9 D 73 A z E tK 1 1 ssignee Na; man odak Company, Rochester, OTHER PUBLICATIONS SomeDemonstrations in O tics Usin a Gas Laser b Button 22 F 1 d. P g Y i 1Feb 1970 et al. Am. J. Phy. 35 355 1964. [21] Appl. No.: 12,083

Primary Examiner-James W. Lawrence Assistant ExaminerD. C. Nelms [52]US. Cl ..250/83.3 H, 250/219 WE, 250/225l3/]2)3F6, Atwmey walter QHodsdon and Robert Cody [51] Int. Cl. ..H0lj 39/00 57 ABSTRACT [58]Field of Search ..25()/217 CR, 202, 83.3 R, 216 R, 1

250/271 R 219 QA 219 FR, 219 WE, 219 DF 219 A flying spot scanner systemis disclosed which obviates the 219 21 DQ, 220, 221, 222, 223, 236; Prir art use of a large expensive lens by reflecting focused 178/76, 7.1;331/945; 95/65; 355/125; 350/7, 99, light off a multifacetedmirror tothe element being scanned. 285 206 The spot 18 made invariant and assmall m size as possible by defining the dimensions of the focused beamso that only part, [56] References Cited preferably half, of a mirrorfacet is illuminated during scanning; and by locating the mirror asclose to the element UNITED STATES PATENTS being scanned as will allowthe spot to be formed of all of the light beamed at' the mirror. Thesource of illumination is a 3,017,512 1/1962 Wolbert ..250/83.3 D laser;and uniform Spot intensity during Scanning is assured by 3,451,7566/1969 Young ..356/237 a Suitably dimensioned stop 3,465,347 9/1969Hudsonm, ....331/94.5 3,360,659 12/1967 Young ..250/236 1 Claim, 4Drawing Figures PATENTEDJUL 4:912 3,675,016

" snm 20F 2 WILLIAM H. ELA/SDELL EDWARD C. CORNELIUS INVENTORS A TTORNEY3 FLYING SPOT SCANNING BACKGROUND OF THE INVENTION 1 Field of theInvention This invention relates generally to optics; and in particular,the invention provides improved optics for purposes of flying spotscanning; and which optics especially serve to control spot size andspot intensity.

2. Description Relative to the Prior Art Good resolution and response ina rotating mirror (multifaceted) flying spot scanner system obtain whenthe spot in question is small, of uniform size during scanning, and ofan effectively constant intensity during such scanning.

Spot size (S) is governed (directly) by the distance (D) of the opticalaperture of the scanner system to the element being scanned; andinversely in proportion to the size (A) of such aperture. If either theparameters D and A are subject to variations, so too will be the spotsize.

The size of the aperture of a rotating mirror scanner system may be keptconstant by either of two general techniques: (1) by directing light, atthe mirror facets, in a beam wide enough to permit, say, several facetsto be simultaneously illuminated; or (2) by directing light, at themirror facets, in a beam that is sufficiently narrow to assure that lessthan a full mirror facet is the most than can ever be illuminated by thebeam, and limiting scanning to that portion of the rotary travel of afacet when such facet is illuminated byall of such light beam. With bothsuch general techniques, the system apertures in question aredimensionally invariant because the rotating facets have no influence onsuch apertures, i.e., they do not themselves become apertures, leavingdefinition of the size of the system aperture to the means which definesthe widths of the beams.

Whereas the first of the above-indicated scanning techniques tends to bewasteful of the available optical energy, i.e., it dilutes such energy,the second of such scanning techniques is not; and it is to such secondtechnique that the invention is especially, but not exclusively,directed:

U.S. Pat. No. 2,844,648 depicts a scanner system of the indicated secondtype, and presumably the scanner system of U.S. Pat. No. 2,844,648 willwork well where the element l l) thereof to be scanned is small inwidth. The problem, however, is how to use a scanner system of the typedisclosed in U.S. Pat. No. 2,844,648, say, to scan webs of 40 or 50inches in width. Absent the invention, this would mean that the spotfocusing lens (21) of U.S. Pat. 2,844,648 would have to be enormous insize; and for the requisite spot dimensions, such lens would have to beprecision-made, and hence would be extremely costly. The term focusing"as used herein means the reduction of one area illumination into asmaller area illumination.

As above noted, a scanner system of the indicated second type is notwasteful of the available optical energy; and there fore readily lendsitself to the rapid inspection of photosensitive webs by means ofinfrared light. As is known, the temporal response and sensitivity ofavailable infrared detectors is less than would otherwise be desired;and it is because of this that the brilliant output of a laser, aspresently preferred, is employed as the light source for spotgeneration.

The light output of a laser in a system of the indicated second type, asimproved by the invention, however, presents still another problem,provided that the effective spot intensity is to be uniform (orreasonably so) during scanning. Such additional problem obtains becausethe output of a laser falls generally gaussianly in proportion to theradial distance from the optical axis of the laser. To image such adistribution of optical energy onto a partial mirror facet is to producea light spot which is, undesirably, brightest at its center, andgradually fades in brightness towards its periphery.

SUMMARY OF THE INVENTION To obviate the prior art requirement, for spotfocusing pur poses in a rotating mirror scanner system, of a (large,precision) lens proximate the (fairly wide) element to be scanned,

the invention proposes to direct focused (i.e., convergent) light (asopposed to collimated light) through a suitably dimensioned stop, andwhich light is focused to a spot on the element to be scanned beforepassing through such stop, there being no need for focusing opticsbetween the rotary mirror and the element being scanned.

More particularly, the invention proposes to define the dimensions ofthe stop in question so that the focused light that passes therethroughcan illuminate only part of a given mirror facet (or parts of adjacentfacets), for power conservation purposes, thereby to assure that thesystem aperture is the fixed aperture of the stop. Such a techniqueassures uniformity in the size of a sharply, but inexpensively produced,scanning spot. And to assure that the focused light spot is as small aspossible, the rotating mirror is positioned as close to the elementbeing scanned as is possible, while still permitting the full width ofsuch element to be scanned; and the dimensions of the focused light beamis so set that approximately half the width of a mirror facet isilluminated by the beam.

Since, as above discussed, a laser-and in particular an in fraredlaser--is a preferred light source for use in rapidly scanning aphotosensitive web, the invention further proposes that the laser outputbeam be so expanded dimensionally by a focusing lens system (i.e., atelescope), prior to the indicated stop, that only the central-fairlyevenly powered-portion of the laser beam is allowed to be focusedthrough the stop, thereby to assure that the focused spot has fairlyconstant power during scanning.

The invention will be further described in relation to the figures, ofwhich FIG. 1 is a schematic diagram illustrating a principal teaching ofthe invention,

FIG. 2 is a diagram, illustrating in exaggeration, not only how focusingis effected, in a scanner system, without a lens proximate the elementbeing scanned, but also illustrating the nonvariability of the effectiveaperture within such system,

FIG. 3 is a diagram useful in describing the relationship between spotsize and facet illumination in a scanner system as proposed by theinvention, and

FIG. 4 illustrates diagrammatically the manner of providing a flyingspot, which spot has substantially even intensity across its dimensions,using a laser.

Referring now to FIG. 1, the invention is cast in the environment of areflection scanner system: a web 10 moves over a rotatable drum 12; anda light spot sweeps across the width of the web 10. The light forproducing the spot is beamed so as to strike the web surface, grazinglywith respect to the periphery of the drum 12 (point P), thereby to bereflected ofl" the web 10 to mirrors 14,16, and thence to aphotodetector 18 for digestion by a using circuit or device 20.Discontinuities and irregularities in the surface of the web 10, atpoint P, cause modulation of the reflected light.

The light spot which sweeps widthwise across the surface of the web 10is, in accordance with the invention, produced as follows: the outputlight beam of a laser 22 is expanded, and then focused to a spot on theweb 10 by means of a telescope lens system 24. Such focused light beamis intercepted by a rotating multifaceted mirror 26 (mirror rotatingmeans not shown), and is thereby caused to sweep across the surface ofthe web 10. To be noted is that a (large, expensive) focusing lens isnot necessary between the rotating mirror 26 and the web pointP-although a lens 27, indicated in phantom, may be employed tocompensate for wobble, etc., of the rotary axis of the mirror, and whichwobble would cause the focusing spot to move periodically in thedirection of the web length.

As above discussed, if the web 10 is photosensitive, it is usuallydesired that inspection of such web be with infrared light. This means(especially if the mirror 26 is rotated at an extremely high rate ofspeed, as is required to maximize the amount of web surface area whichis actually inspected) that an extremely high intensity spot be focusedonto the web, for otherwise the photodetector 18, which in this case isan infrared detector, will be substantially unresponsive to lightmodulation by surface defects. Infrared detectors are notoriously low intemporal response and sensitivity. To economize on the available powerfor spot generation purposes, apparatus according to the inventionpurposely confines the focused light within a beam which is sodimensioned widthwise that only part of any given mirror facet (or partsof adjacent facets) can ever be illuminated at a given time. This meansthat all of the source light is, while the focused spot is on the web10, assuredly directed at the web, and nowhere else.

FIG. 2 illustrates how a beam, focused to a spot on the web 10, producesa spot that has uniform dimensions during the transversal of the spotacross the web: This means, assuming the mirror-to-web distance R isfixed, that the system aperture A is of a constant size, i.e., themirror facets do not become apertures which gradually increase in sizeto a certain amount, and then gradually decrease in size. For a givenamount of widthwise facet illumination, the width of the web 10 to bescanned determines the distance R that the mirror 26 must be for a spotwhich is of uniform size during scanning. However, the greater theamount of facet illumination, the farther the rotating mirror 26 must befrom the web 10 to produce a spot which is of uniform size duringscanning. As is above indicated, spot size depends, directly, on thesize of the system aperture and, inversely, on the distance of suchaperture from the web. FIG. 3 illustrates that if the facet illuminationis small i.e., small system aperture)thereby allowing the mirror-towebdistance R to be small-the spot size will be large because the systemaperture will be controlling of spot size. Similarly, if the facetillumination is large (large system aperture), the mirror-to-webdistance R will have to be great if a spot of uniform size is to beswept across the web 10; and because of such great distance, the spotsize will, attendantly, be large. To effect a uniformly sized spot,which is also small in size, the in vention indicates that the widthwisemirror facet illumination be (approximately) 50 percent, and that themirror 26 be positioned as close to the web 10 as is necessary to assurethat the mirror facets do not become varying apertures, whichcorrolarily vary spot size.

The fixed system aperture, which obtains during web scanning, isprovided by means of a stop 28. The stop 28, in combination with theexpanding telescope 24, serves to produce a spot of substantiallyuniform intensity across its dimensions: As is known, the power outputof a laser falls gaussianly in relation to radial distance from theoptical axis of the laser. Thus, with the stop 28 so dimensioned thatits output can illuminate only about one-half of any given mirror facet,the telescope 24-while focusing the laser output to a spot on the webl-so expands dimensionally such output that laser light which isradially well away from the optical axis of the laser gets blocked bythe stop 28. See FIG. 4 which graphically indicates how spot intensityis kept constant during scanning by blocking off-axis laser light.

Summarizing the above, it may be said that the invention teaches:

1. that in a rotating mirror flying spot scanner system, expensive largefocusing lenses can be obviated provided source light is focused to aspot on the element being scanned by means disposed on the source sideof the rotating mirror,

2. that spot size may be made uniform by so focusing such light thatonly part of a mirror facet can ever be illuminated, and by so settingthe mirror scanning distance that the spot is formed on the elementbeing scanned only when all of the source light is reflected by themirror to the element,

3. that spot size may be made as small as possible by so defining thewidthwise dimensions of the focused beam that it can illuminate onlyone-half of a given mirror facet, and

4. that, with use of a laser in a scanner system, spot intensity may bemade fairly uniform by suitable defining the dimensions of a stop, andby so expanding, and focusing, the output beam from the laser thatfringe illumination is blocked by the l he invention has been describedin detail with particular reference to preferred embodiments thereof,but it will be understood that variations and modifications can beeffected within the spirit and scope of the invention.

What is claimed is: 1. In a flying spot scanner system of the typehaving: a. a multifaceted mirror adapted for rotation about its rotaryaxis, b. an infrared laser source of light, and 0. means for beamingsaid light at said mirror for reflection thereof to a photosensitive webbeing scanned, the improvement wherein the said means for beaming saidinfrared light is adapted to focus said infrared light to a spot on thesaid element being scanned before said infrared light strikes the saidmirror for reflection to the said photosensitive web, said systemincluding means for so restricting the dimensions of the focusing beamstriking the said mirror that only part of a mirror facet is illuminatedduring substantially all of the time that the said light is reflected tothe element being scanned,

the focused beam being so dimensioned that it illuminates approximatelyone-half a mirror facet during the time that such beam is whollyreflected from such facet, and the distance (D) of said mirror from saidelement being scanned being substantially as small as is necessary forthe defined spot to be formed during scanning from substantially all ofthe infrared light beamed at the said mirror,

a. photodetector means responsive to infrared radiation for detectinglight from the said spot,

b. lens means for expanding the dimensions of the output beam from saidlaser, and for focusing said beam via said mirror to a spot on the saidphotosensitive web being scanned, and

c. means for providing an optical stop for blocking light from the saidlaser which is radially more than a predetermined distance off theoptical axis of the said laser.

1. In a flying spot scanner system of the type having: a. a multifacetedmirror adapted for rotation about its rotary axis, b. an infrared lasersource of light, and c. means for beaming said light at said mirror forreflection thereof to a photosensitive web being scanned, theimprovement wherein the said means for beaming said infrared light isadapted to focus said infrared light to a spot on the said element beingscanned before said infrared light strikes the said mirror forreflection to the said photosensitive web, said system including meansfor so restricting the dimensions of the focusing beam striking the saidmirror that only part of a mirror facet is illuminated duringsubstantially all of the time that the said light is reflected to theelement being scanned, the focused beam being so dimensioned that itilluminates approximately one-half a mirror facet during the time thatsuch beam is wholly reflected from such facet, and the distance (D) ofsaid mirror from said element being scanned being substantially as smallas is necessary for the defined spot to be formed during scanning fromsubstantially all of the infrared light beamed at the said mirror, a.photodetector means responsive to infrared radiation for detecting lightfrom the said spot, b. lens means for expanding the dimensions of theoutput beam from said laser, and for focusing said beam via said mirrorto a spot on the said photosensitive web being scanned, and c. means forproviding an optical stop for blocking light from the said laser whichis radially more than a predetermined distance off the optical axis ofthe said laser.